Ultrafast self-heating synthesis of robust heterogeneous nanocarbides for high current density hydrogen evolution reaction

Designing cost-effective and high-efficiency catalysts to electrolyze water is an effective way of producing hydrogen. Practical applications require highly active and stable hydrogen evolution reaction catalysts working at high current densities (≥1000 mA cm−2). However, it is challenging to simultaneously enhance the catalytic activity and interface stability of these catalysts. Herein, we report a rapid, energy-saving, and self-heating method to synthesize high-efficiency Mo2C/MoC/carbon nanotube hydrogen evolution reaction catalysts by ultrafast heating and cooling. The experiments and density functional theory calculations reveal that numerous Mo2C/MoC hetero-interfaces offer abundant active sites with a moderate hydrogen adsorption free energy ΔGH* (0.02 eV), and strong chemical bonding between the Mo2C/MoC catalysts and carbon nanotube heater/electrode significantly enhances the mechanical stability owing to instantaneous high temperature. As a result, the Mo2C/MoC/carbon nanotube catalyst achieves low overpotentials of 233 and 255 mV at 1000 and 1500 mA cm−2 in 1 M KOH, respectively, and the overpotential shows only a slight change after working at 1000 mA cm−2 for 14 days, suggesting the excellent activity and stability of the high-current-density hydrogen evolution reaction catalyst. The promising activity, excellent stability, and high productivity of our catalyst can fulfil the demands of hydrogen production in various applications.

MoC/Mo2C/CNT. It is also important for the authors to measure the polarization curves of a physical mixture of MoC/CNT + Mo2C/CNT, since the physical mixture has no Mo3+ and MoC/Mo2C interfaces. Should the MoC/Mo2C interfacial area be truly important, the physical mixture will not perform as well as MoC/Mo2C/CNT. Additionally, the authors can calculate the turnover frequency for all 3 variants to illustrate that the activity increase is intrinsic. • Line 244-246 and supplementary tab 2: the authors should include all the pre-and post-synthesis processing time, including the time it takes to create holes in the CNT. Comparing the time required to synthesize a catalyst might not be entirely fair too, given that different techniques can produce catalysts at different mass scales. The authors can emphasize on their rapid synthesis process, which is a marked improvement from many calcination/slow thermal treatment processes. • Line 278-281: Authors can try using a graphite counter electrode as a control since they have identified dissolved Pt counter electrode as a possible interference, especially since they are running HER at such high current densities • Line 297: Authors should give quantitative measures on the intensity of the MoC : Mo2C peaks after stability testing, just like how they provided in Fig 2c (right) • Line 322-325 and Fig 4g: Authors will need to verify with all the other papers that the same electrolyte was used for stability testing. It will not be fair to compare HER in alkali (this paper) to HER in acid and especially neutral electrolytes, since the mechanism of HER differs depending on pH.
• Line 356-358: Authors claim that defects and dislocations further enhance HER. What is the proportion of HER activity attributed to defects compared to the MoC/Mo2C interface? • Line 372-374: Authors will need to cross-check their synthesized Nb2C and W2C compounds against available data for Nb2C and W2C MXenes before claiming that they produced 2D MXenes. Similar problem to Mo2C on the first point.
Minor comments: • Line 45: Pt-group metals (Ru, Rh, Pd, Ir, Os, Pt) all have very high HER activity and the authors can consider rephrasing non-noble metal as Pt-group metal-free. • Line 49: Authors suggest that a highly stable catalyst require chemical and mechanical considerations, while a highly active catalyst requires only chemical considerations. By following this statement, wouldn't designing a stable catalyst automatically produce an active catalyst? I would recommend the authors rephrase this sentence for clarity, as there is no opposition in this sentence. • Line 52-56: citation required to exemplify problem of catalyst exfoliation from HER • Line 65-66: citation required to exemplify problem of binders obstructing catalytic sites • Line 74-77: should include "in the presence of Mo and C precursors" to synthesize the film, as CNTs do not provide the Mo or C precursors for film formation • Line 130 and Fig 2c: authors should increase the separation between both graphs as "40" from the left graph is too close to "0.6" from the right graph • Lin 155: "By combining XPS with a total weight loss" sounds awkward. It might help to rephrase as "combining XPS and TGA data". • Line 224: Authors will need to specify 20 wt% of Pt in Pt/C in This is an interesting paper on the development of MXene/CNT catalysts for hydrogen evolution. The work has been carried out quite well, the materials have been characterised well and the electroanalysis is reasonably good. I do have reservations about the use of double layer capacitance as a measure of the electrochemical surface area of the materials. The specific double layer capacitance of these materials is not known very accurately and this will lead to a high degree of error in the reported values. Following on from that point, I note there is almost no consideration given to error analysis in the paper. Some estimates of the errors in the reported quantities should be given.
All of the above aside, I am unfortunately not convinced that this paper reveals sufficiently new insights for publication in the journal. There are very many example of materials that are similar to those reported here (differences in preparation methods notwithstanding). I am not convinced that the reported values are of sufficient importance for the readership.
Reviewer #3 (Remarks to the Author): Title: Ultrafast self-heating synthesis of robust MXene/CNT catalysts for stable high-current-density hydrogen evolution reaction Recommendation: minor revision In this paper, CNT was used as matrix and heat source to in situ synthesize Mo2C/MoC/CNT catalysts for highly active and stable HER process. The ultrafast heating and cooling rate and short growth time benefited the formation of active sites. The Mo2C/MoC hetero-interface enhanced electron exchange, resulting in the formation of Mo3+ sites serve as the main HER active sites. The strong coupling between the MoxC and CNT matrix ensure the long-term stability at high current densities of catalysts.
Here are some suggestions that were shown below for the improvement of manuscript quality.
1. To improve the electrode stability, the CNT film was drilled by laser for H2 release channels. Please describe the specific experimental steps of laser drilling. 2. It was mentioned in this paper that Mo3+ sites served as the main HER active sites. Please provide sufficient evidence that Mo3+ has high HER activity. 3. In the mechanical tensile experiments, whether the improvement of material mechanical strength was related to the rapid rise and fall of joule heating? Please add to prove the comparison sample of pure CNT film heated by joule heating. 4. As shown in Fig. 4e, although the f-Mo2C/MoC/CNT catalyst deteriorated after 6 days of test, the potential did not change significantly. Please supplement the data of f-Mo2C/MoC/CNT catalyst tested after 14 days and compare it with the Mo2C/MoC/CNT catalyst prepared by self-heating. 5. In supplementary Fig. 23, the f-Mo2C/MoC/CNT catalyst showed the XRD diffraction peak of MoO3 after 6 days of test, while the Mo2C/MoC/CNT catalyst prepared by self-heating did not show the MoO3 peak. Please explain why the self-heating sample has better stability.
[Comment] Li et al. report a self-heating method to synthesize Mo2C MXene/MoC/CNT for highly efficient HER with long durability. DFT calculations reveal an almost thermoneutral H* binding energy, optimal for HER activity. The HER performance and stability of the synthesized catalyst is very attractive and one of the highest I have seen, but more work needs to be done to fully characterize (1) the identity of the catalyst formed and (2) the nature of the catalyst interface. I am unsure if the Mo2C formed is a 2D MXene, and if indeed Mo 3+ exists at the interface which is responsible for the high HER activity.
[Response] We thank the reviewer for all of the comments on our manuscript. As shown in the following responses, we have supplemented more detailed and systematic data in the revised manuscript to make our work more solid. We hope that the reviewer finds our manuscript suitable for publication in Nature Communications now.
[Comment] Main comments: • Authors claim that Mo2C formed is a 2D MXene in the title. This might be true but the authors will have to provide evidence that the Mo2C sheets formed are 2D (by AFM) and further characterize them. The authors have used Mo2C nanoparticles instead of Mo2C MXene reference data for the XRD, Raman, XPS analysis. It is more likely than not that the authors have formed Mo2C nanoparticles (which they will have to identify which phase it is) and not 2D MXene sheets. Their Mo2C XRD reference (line 125, Fig 2b) is not Mo2C MXene (Adv. Funct. Mater. 2016, 26, 3118-3127), and their XRD peak (Fig 2b) (Adv. Funct. Mater. 2016, 26, 3118-3127) while the authors have Mo 2+ peaks in their Mo2C sample, which suggests that their Mo2C formed might not be Mo2C MXene. Mo2C was also grown within the CNT pores and have small particle sizes (Fig 3c inset), which is in opposition to the main characteristic of MXenes, which are flat large-area 2D materials (Adv. Mater. 2017, 29, 1700072). Fig 3c shows that Mo2C exists more like a particle than a 2D material, thus calling Mo2C a MXene might not be appropriate.

[Response]
We appreciate this valuable comment. Following the reviewer's suggestion, we remeasured the X-ray diffraction (XRD) of the self-heating samples in a wider range of 2 to further identify the catalyst formed. As reported in the literature (Nat. Commun. 12, 5510 (2021);Adv. Funct. Mater. 26, 3118-3127 (2016)), there exists an XRD peak of Mo2C MXene at 2~9° and a c lattice parameter of 20.6 Å. Figure R1 shows that in our sample, there is a weak peak at about 11.5°, corresponding to a reduced c lattice parameter, which is attributed to a transition phase from Mo2C MXene to β-Mo2C. This may result from the defunctionalization of the Tx groups (Nat. Commun. 12, 5510 (2021);Chem. Mater. 31, 4505-4513 (2019)), the removal of intercalated water, and the reestablishment of long-range order in Mo2C MXene. Because the self-heating synthesis involves an extremely high-temperature process and a reducing H2 atmosphere, the change of the c lattice parameter is reasonable. In the XRD spectrum of the Mo2C/MoC/CNT film, other stronger peaks originate from β-Mo2C and α-MoC (Fig. R1), which indicates that β-Mo2C and α-MoC are major phases in the composite and the transition phase from Mo2C MXene to β-Mo2C is minor. We agree with the reviewer that the Raman spectra, the XPS spectra, and the particle morphology also reveal the dominant β-Mo2C and α-MoC phases in the composite.
In response to this comment, we added Fig. R1 as new Supplementary Fig. 3, cited the references suggested, revised "MXene/CNT catalyst" into "nano-carbides/CNT catalyst" in the title, and added the above discussion on Page 7 of the revised manuscript.
[Comment] • Lines 93-99 and materials section: Authors describe the synthesis procedure using step 1, 2, 3 but the steps are not labelled on Fig 1a. Authors should also mention in which gas environment the films are dried in for step 2, and the gas flow rates for step 3. Authors will need to be more specific in the methods section for readers to reproduce the work: what pH is "alkaline" in line 426, how many times is "several" in line 427/439, and what is "certain proportion" in line 424? It will also help readers if the authors can expand on Fig 1c to include the 30 W and 135 W heating regions and time as a full temperature ramp profile, instead of truncating the profile to show only the rapid heating/cooling at the start/end. Authors can also briefly describe the self-heating phenomena in CNTs in lines 93-99.

[Response]
We thank the reviewer for these comments in details. We respond to these comments point by point as follows.
(1) We labeled the steps in Fig. 1a of the revised manuscript (also see Fig. R2). (2) We specified the gas environment in step 2 and the gas flow rates in step 3 on Page 5 of the revised manuscript: "Second, the CNT film loaded with precursors was dried at 60°C for 10 min in air (step 2). Finally, self-heating synthesis was performed in a mixed atmosphere of 10% H2 and 90% Ar with a total flow rate of 200 sccm, in which the precursors in situ reacted on the rapidly Joule-heated CNT film (step 3)".
(3) We specified the pH of alkaline, the times, and the proportion, etc. in the Methods section of the revised manuscript: "To synthesize Mo2C/MoC/CNT film, ammonium molybdate ((NH4)2MoO4·4H2O) and glucose (C6H12O6) with varied atomic ratios of Mo:C (1:1, 4:3, 2:1, and 4:1) were dissolved in a mixed solution of deionized water and ethanol. Urea (CH4N2O) was then dissolved into the mixed solution based on an optimal atomic ratio of Mo:C (4:3 in the experiment) and varied mole ratios of glucose to urea (0, 10:1, 20:3, 5:1, 5:2). To promote the dissolution of (NH4)2MoO4·4H2O and prevent the solution from precipitation, we added ammonia water to adjust the pH value of the solution to about 11.5. The precursor was loaded on a CNT film through three times of dip coating and then dried at 60°C for 10 min in air." (4) In Fig. 1c, we would like to show the advantage of the rapid heating/cooling in our selfheating method, which allows the control of the entire growth process within a short time. Following the reviewer's suggestion, we also presented a full temperature ramp profile for a heating process of 30 s at 30 W followed by the other process of 45 s at 135 W, as shown in Fig. R3. The heating and cooling processes in Fig. R3 are also on the order of hundreds of milliseconds. We added this figure as Supplementary Fig. 1a.    The threshold voltage per unit length applied for the CNT films emitting visible light is ~ 0.5 V/mm. We have specified this voltage value on Page 5 of the revised manuscript.
[Comment] • Line 130 and Fig 2c: authors should increase the separation between both graphs as "40" from the left graph is too close to "0.6" from the right graph.

[Response]
We have separated the graphs in Fig. 2c to avoid the crowding of both x-axes. The new Fig. 2c is shown as follows. [Comment] • Line 144: authors should provide evidence that Mo 3+ mixed valency and electron transfer exists solely at the MoC-Mo2C interface (and not MoC or Mo2C regions) to justify the lowered Mo 4+ BE and increased Mo 2+ BE.

[Response]
We thank the reviewer for this valuable comment. Following the reviewer's suggestion, we provided more experimental and theoretical evidence to support the electron transfer scenario at the Mo2C/MoC interface.
First, we carefully reviewed the valence states of Mo in Mo2C and MoC reported in the literature and re-analyzed the change of valence states in our materials. It is still debating for the exact valence states of Mo in Mo2C or MoC. Most studies suggest the dominance of Mo 2+ in Mo2C and Mo 3+ in MoC, and higher valence states of Mo 4+ , Mo 5+ , and Mo 6+ result from partial oxidation (Nat. Catal. 1, 960-967 (2018); Nat. Commun. 12, 6776 (2021); J. Am. Chem. Soc. 140, 14481-14489 (2018)). Under this assumption, we re-analyzed the XPS spectra. As shown in Fig. R6, compared with the Mo 2+ peak in Mo2C and the Mo 3+ peak in MoC, in the Mo2C/MoC/CNT film the Mo 2+ peak apparently blueshifts while the Mo 3+ peak redshifts, suggesting the existence of electron transfer from Mo2C to MoC in the heterogeneous composite. Note that the strong Mo 4+ , Mo 5+ , and Mo 6+ peaks in the MoC sample should result from the surface MoOx because MoC is very prone to oxidation.  In response to this comment, we added Fig. R6 and R7 as new Fig. 2d and Supplementary Fig. 8. We also added the above response on Page 8 of the revised manuscript.
[Comment] • Line 145: authors will need to reason why Mo 3+ and not Mo 2+ or Mo 4+ are HER active. It will also help if authors can provide the peak area ratio of Mo 2+ : Mo 3+ : Mo 4+ for comparison since the authors will use the XPS data to estimate the CNT : MoC : Mo. Also, given that the CNT wt ratio is 61.8%, will it be useful for the authors to reduce the CNT wt % and increase MoC/Mo2C interfacial area for HER?

[Response]
We thank the reviewer for this valuable comment. As we re-analysed in the last response, we note that the dominant valence state of Mo is +2 in Mo2C and +3 in MoC mainly. The exact valence state of Mo at the Mo2C/MoC interface may be between +2 and +3 due to the electron transfer over there. Under such a scenario, it is not possible to use the peak area ratio of Mo 2+ : Mo 3+ : Mo 4+ as an indicator to compare the HER activity.
To reveal why the Mo2C/MoC interface are more active, we performed further DFT calculations to investigate the electronic structures of MoC (111)  We also varied the CNT wt% by loading different amounts of precursors on CNT films. The CNT wt% in the Mo2C/MoC/CNT samples prepared from precursors loaded by 1, 3, and 5 times is about 83.0%, 61.8%, and 49.3%, respectively. As shown in Fig. R9, the sample with the moderate CNT wt% (~ 61.8 wt% CNT and ~ 38.2 wt% Mo2C/MoC) possesses the best HER activity. It exhibits no apparent agglomeration of nanoparticles and thus has abundant Mo2C/MoC interfacial area (Fig.  R10b). More or less content of Mo2C/MoC interfaces will lead to poorer HER activity. When the content of CNT is high, the content of MoC or Mo2C is very few and thereby the total Mo2C/MoC interfacial area becomes very limited (Fig. R10a). And for the samples with a low content of CNT, Mo2C/MoC will agglomerate during the self-heating process, also reducing the total MoC/Mo2C interfacial area (Fig. R10c). This result suggests that an appropriate CNT wt% is needed to increase the Mo2C/MoC interfacial area for a better HER.   [Response] We thank the reviewer for this valuable comment. The heating ramp conditions do affect the size distribution of the Mo2C/MoC nanoparticles. To show that, we changed the heating ramp time for both heating steps from room temperature to ~ 1100 K (30 W) and from ~ 1100 to ~1770 K (135 W). As the heating ramp rate decreases, the particles gradually agglomerate and increase in size because they are more likely to diffuse and aggregate during the heating process, especially at high temperatures (Fig. R11).  In doing so, the authors might see that the performance of MoC/CNT and Mo2C/CNT will be closer to MoC/Mo2C/CNT. It is also important for the authors to measure the polarization curves of a physical mixture of MoC/CNT + Mo2C/CNT, since the physical mixture has no Mo3+ and MoC/Mo2C interfaces. Should the MoC/Mo2C interfacial area be truly important, the physical mixture will not perform as well as MoC/Mo2C/CNT. Additionally, the authors can calculate the turnover frequency for all 3 variants to illustrate that the activity increase is intrinsic.

[Response]
We thank the reviewer for these comments. We supplemented more experiments and analyses to show the ECSA-normalized polarization curve, the effect of the physical mixture, and the turnover frequency.
First, the ECSA of each sample can be evaluated from the double-layer capacitance (Cdl) according to where Cs is the specific capacitance of the sample or the capacitance of an atomically smooth planar surface of the material per unit area under the same condition. Cs for a flat surface is generally found to be in the range of 20-60 μF cm -2 (Nat. Energy 4, 512-518 (2019); Nat. Mater. 18, 1309-1314 (2019)), and the value of 40 μF cm -2 is adopted in this work to calculate the ECSA.
The ECSAs of Mo2C/MoC/CNT film, Mo2C/CNT film, and MoC/CNT film are calculated to be 2998, 2088, and 1650 cm 2 , respectively, according to previous CV results. The polarization curves of three films normalized to ECSA are re-plotted in Fig. R12, which demonstrates that the activity of Mo2C/MoC/CNT film does increase intrinsically compared with those of Mo2C/CNT film and MoC/CNT film. Second, we found that it was very difficult to disperse Mo2C/CNT film and MoC/CNT film and physically mix them. Instead, we mechanically mixed the MoC and Mo2C powders, dispersed them in the ethanol/water/Nafion solution, and dropped the solution onto a CNT film to prepare a physically mixed Mo2C/MoC/CNT film (m-Mo2C/MoC/CNT film). Because of the lack of the chemical bonding interaction and the efficient charge transfer between Mo2C and MoC, the m-Mo2C/MoC/CNT film has a much poorer HER performance than the Mo2C/MoC/CNT film prepared by the self-heating process (Fig. R13), which elucidates that the Mo2C/MoC interface is truly important. Moreover, we also evaluated the turnover frequency (TOF) of each catalyst film (Fig. R14). The TOF is calculated by the following equation (Nat. Energy 4, 512-518 (2019); Nat. Commun. 10, 3755 (2019)): where N is the density of active sites, n is the number of electrons involved in the reaction. The density of active sites can be calculated as follows, At an overpotential of 250 mV, the TOF of Mo2C/MoC/CNT film, Mo2C/CNT film, and MoC/CNT film is 0.65 s -1 , 0.30 s -1 , and 0.22 s -1 , respectively, which validates that Mo2C/MoC/CNT catalyst has higher intrinsic activity besides the larger ECSA. The increase of the intrinsic activity should be attributed to the abundant Mo2C/MoC interfaces. In response to this comment, we added Fig. R12, R13, and R14 as Supplementary Fig. 23, Supplementary Fig. 19, and Supplementary Fig. 24, respectively, and the above response on Page 14 and 15 of the revised manuscript.
[Comment] • Line 244-246 and supplementary tab 2: the authors should include all the pre-and post-synthesis processing time, including the time it takes to create holes in the CNT. Comparing the time required to synthesize a catalyst might not be entirely fair too, given that different techniques can produce catalysts at different mass scales. The authors can emphasize on their rapid synthesis process, which is a marked improvement from many calcination/slow thermal treatment processes.

[Response]
We thank the reviewer for this valuable comment. In our method, the pre-synthesis processing includes CNT film preparation, laser drilling, and precursor loading, which takes about 15 minutes in total. After a self-heating synthesis at high temperature, the as-prepared composite film can be served as an electrode directly. Our method costs a comparable pre-/post-synthesis processing time and a much shorter synthesis time than the traditional methods. However, since the authors did not clearly describe the time required for pre-and post-processing using other methods in the literature, it is difficult to directly compare the time required to synthesize a catalyst, including all the pre-and post-synthesis processing time.
In response to the comment, we listed the "synthesis time" excluding the pre-and post-synthesis processing time in Supplementary Table 2, and noted the pre-/post-synthesis processing time is ~ 15 minutes below the table. We described the table on Page 15 of the revised manuscript as follows: "Besides the excellent HER activity of the Mo2C/MoC/CNT film at high current densities, our selfheating method has notable advantages in the rapid synthesis process and high productivity, which takes a comparable pre-/post-synthesis processing time and a much shorter synthesis time than traditional methods ( Fig. 4b and Supplementary Tab. 2)."

[Comment]
• Line 278-281: Authors can try using a graphite counter electrode as a control since they have identified dissolved Pt counter electrode as a possible interference, especially since they are running HER at such high current densities [Response] We thank the reviewer for this suggestion. We tried to use a graphite counter electrode (Gaoss Union) in our stability measurement under a large current density. However, the graphite counter electrode was found to be dissolved in the KOH solution after the test at a current density of 1000 mA cm -2 for ~4 days, causing the interruption of the measurement (Fig. R15). In spite of this, the unchanged overpotential during the first 4 days still reveals the high stability of our Mo2C/MoC/CNT film. Actually, many previous studies used Pt as the counter electrode in the high-current-density HER (Nat. Energy 4, 107-114 (2018); Nat. Mater. 18, 1309Mater. 18, -1314Mater. 18, (2019; Nat. Commun. 9, 2609Commun. 9, (2018). In our experiment, after a long-term test at a high current density, the content of Pt in the electrolyte was below the detection limit of inductively coupled plasma (ICP) mass spectrometry, which excludes the influence of Pt dissolution from counter electrodes during the electrochemical measurements (Supplementary Tab. 4).
We also used a graphite rod as the counter electrode to measure the CV curves of Mo2C/MoC/CNT films for 50 cycles, and then changed to use a Pt counter electrode for the other 50-cycles CV measurement. The CV curves obtained by graphite and Pt counter electrodes are identical (Fig. R16), also verifying that the Pt counter electrode does not influence the results. In summary, we found that graphite rods are not suitable as counter electrodes in long-term tests at high current density. Therefore, we used Pt instead of graphite rods as the counter electrodes for the stability measurement. In response to this comment, we added Fig. R15 and R16 as Supplementary Fig. 28 and Fig. 29, respectively, and the above response on Page 17 of the revised manuscript.
[Comment] • Line 297: Authors should give quantitative measures on the intensity of the MoC : Mo2C peaks after stability testing, just like how they provided in Fig 2c (right) [Response] We thank the reviewer for this valuable comment. We quantitatively measured the XRD peak intensity of MoC (111) and Mo2C (101) after the stability test according to Supplementary Fig.  33. The peak intensity ratio is MoC (111) : Mo2C (101) ~ 1.4 after the test, which is higher than its pristine value (~ 0.7) (Fig. R17). The reason may lie in the fact that Mo2C (101) is more susceptible to corrosion than MoC (111). Although the material is corroded, the remained excellent HER performance supports our statement that the Mo2C/MoC interface, rather than single MoC or Mo2C, is crucial in the HER activity.
In response to this comment, we added Fig. R17 as Supplementary Fig. 33b and the above discussion on Page 18 of the revised manuscript.  Authors will need to verify with all the other papers that the same electrolyte was used for stability testing. It will not be fair to compare HER in alkali (this paper) to HER in acid and especially neutral electrolytes, since the mechanism of HER differs depending on pH.
[Response] We thank the reviewer for this suggestion. In Fig. 4g, almost all catalysts for comparison are measured in alkali with pH=14. We agree with the reviewer that the mechanism is different in alkali/acid/neutral electrolytes, but high stability of the catalytic electrode is required regardless of the electrolyte type.
In response to this comment, we marked the type of electrolytes in Fig. 4g (also see Fig. R18), and revised the description for the comparison on Page 20 as follows, "As shown in Fig. 4g and Supplementary Tab. 5, the Csta of the Mo2C/MoC/CNT film is as large as 2.57 × 10 7 C cm -2 V -1 , which is times or even orders of magnitude higher than the values of other high-performance HER catalysts including 1T-MoS2, MoNi4/MoO2, Ni2P-Fe2P, Co-NiS2, CoP/NiCoP/NC, S-MoS2@C, single atom NiI, etc. that were measured in alkali, as well as IrFe/NC and Mo2CTx/2H-MoS2 that were measured in acid." [Comment] • Line 356-358: Authors claim that defects and dislocations further enhance HER. What is the proportion of HER activity attributed to defects compared to the MoC/Mo2C interface?

[Response]
We thank the reviewer for this valuable comment. By using DFT calculations, we investigate the vacancy formation energies of Mo and C. We reveal that C vacancy is easier to form than Mo vacancy for both Mo2C and MoC surfaces (Table R1). Thus, we further investigate the ΔGH* of the Mo2C (100), MoC (111) surfaces with C vacancy, as shown in Fig. R19. DFT calculations show that the absolute values of ΔGH* for Mo2C (100) surface with C vacancy are almost unchanged compared to those of Mo2C (100) surface without C defects, showing the poor HER activity. However, the absolute value of ΔGH* for MoC (111) surface with C vacancy decreases and is closer to 0 eV than that of MoC (111) surface without C defects, showing that the carbon vacancy defects in the MoC (111) surface can improve HER activity.
Although C vacancy may promote HER activity, the ΔGH* for Mo2C/MoC interface is as low as 0.02 eV, which is closer to 0 eV than that of Mo2C (-0.35 or 0.65 eV for (100)) and MoC (-0.04 eV for (111)) surfaces with the C vacancy. In experiments, the HER performance of the Mo2C/MoC composite is much better than the single Mo2C or MoC phase. Therefore, in the Mo2C/MoC/CNT film, we believe that the Mo2C/MoC interface is the main contribution to the HER activity, and the defects may only additionally promote the HER activity.
In response to this comment, we added Table R1   compounds against available data for Nb2C and W2C MXenes before claiming that they produced 2D MXenes. Similar problem to Mo2C on the first point.

[Response]
We thank the reviewer for this valuable comment. Similar to the analysis of Mo2C/MoC/CNT film as mentioned above, Nb4C3 and W2C/WC composite contains mainly 3Dlattice structures (Fig. R20, also as Supplementary Fig. 40). For rigorous expression, we described them as nano-carbides in the revised manuscript. [Comment] Minor comments: • Line 45: Pt-group metals (Ru, Rh, Pd, Ir, Os, Pt) all have very high HER activity and the authors can consider rephrasing non-noble metal as Pt-group metal-free.

[Response]
We thank the reviewer for this valuable comment. Following the suggestion, we revised the description of "non-noble metal" as "Pt-group metal-free" throughout the manuscript.
[Comment] • Line 49: Authors suggest that a highly stable catalyst require chemical and mechanical considerations, while a highly active catalyst requires only chemical considerations. By following this statement, wouldn't designing a stable catalyst automatically produce an active catalyst? I would recommend the authors rephrase this sentence for clarity, as there is no opposition in this sentence.

[Response]
We thank the reviewer for this valuable comment. We rephrase this sentence on Page 3 of the revised manuscript as follows: "However, the development of high-efficiency and Pt-group metal-free HER catalytic electrodes for high-current-density HER is challenging, because it requires simultaneous high chemical activity, high chemical stability, and high mechanical stability of the electrodes."  (2019)) as Ref. 14, 23, and 24 to exemplify the problem of binders.
[Comment] • Line 74-77: should include "in the presence of Mo and C precursors" to synthesize the film, as CNTs do not provide the Mo or C precursors for film formation

[Response]
We revised this sentence into "we develop a low-energy-consumption method using a carbon nanotube (CNT) film as a heat source and matrix, which rapidly changes its temperature in hundreds of milliseconds to in situ synthesize a robust Mo2C/MoC/CNT composite film in the presence of Mo and C precursors." on Page 4 of the revised manuscript.
[Comment] • Line 130 and Fig 2c: authors should increase the separation between both graphs as "40" from the left graph is too close to "0.6" from the right graph

[Response]
We adjusted the separation in Fig. 2c as suggested.
[Comment] • Lin 155: "By combining XPS with a total weight loss" sounds awkward. It might help to rephrase as "combining XPS and TGA data".
[Response] Following the suggestion, we rephrased this description into "combining XPS and TGA data". We thank the reviewer for his/her carefulness. We have corrected this typo.

Reviewer #2 (Remarks to the Author): [Comment] This is an interesting paper on the development of MXene/CNT catalysts for hydrogen evolution. The work has been carried out quite well, the materials have been characterised well and the electroanalysis is reasonably good. I do have reservations about the use of double layer
capacitance as a measure of the electrochemical surface area of the materials. The specific double layer capacitance of these materials is not known very accurately and this will lead to a high degree of error in the reported values. Following on from that point, I note there is almost no consideration given to error analysis in the paper. Some estimates of the errors in the reported quantities should be given.

[Response]
We appreciate the reviewer's evaluation and valuable comments on our manuscript. In the revised manuscript, we have supplemented more detailed and systematic data to make our work more solid, and further highlighted the novelty of our work.
In response to the specific comment, we would like to note that the electrochemical surface area (ECSA), which is proportional to the double-layer capacitance (Cdl), is usually used to estimate the density of active sites (Nat. Commun. 11, 5462 (2020); Sci. Adv. 5, eaav6009 (2019)). The doublelayer capacitance (Cdl) is obtained by fitting the relationship between the variation of currents and the sweeping rate at a fixed potential in the non-Faradaic region, because in this region almost all currents come from the capacitance effect. This method is widely adopted in other works (for example, Nat. Energy 4, 512-518 (2019); Nat. Mater. 18, 1309-1314 (2019)).
The ECSA of a material can be calculated from the double-layer capacitance (Cdl) by the following equation, where Cs is the specific capacitance of the material or the capacitance of an atomically flat surface per unit area under the same condition.
For Cdl, the measurement error includes systematic errors (0.01% for voltage application and 0.2% for current detection) and random error (~5%), which is negligible. For Cs, its value for a flat surface is generally found to be in the range of 20-60 μF cm -2 (Nat. Energy 4, 512-518 (2019); Nat. Mater. 18, 1309-1314 (2019)), and the value of 40 μF cm -2 is adopted in this work to calculate the ECSA. As the actual Cs cannot be determined precisely in this work, this treatment may cause an error in the absolute value of ECSA. However, the relative values of the ECSA for the materials reported in this work are not affected by the absolute error of Cs, because the Mo2C/MoC/CNT film, Mo2C/CNT film, and MoC/CNT film are prepared on the same supporting material, the CNT films. The Cs values for all these materials should be nearly identical in principle, and thus the calculated ECSAs can be compared relatively. With the specific Cs of 40 μF cm -2 , the ECSAs of Mo2C/MoC/CNT film, Mo2C/CNT film, and MoC/CNT film are calculated to be 2998, 2088, and 1650 cm 2 , respectively. The polarization curves of the three films are re-plotted in Fig. R12, which demonstrates that the activity of Mo2C/MoC/CNT film does increase intrinsically compared with that of Mo2C/CNT film and MoC/CNT film.
In response to this comment, we have added Fig. R12 as Supplementary Fig. 23, and part of the above discussion on Page 15 of the revised manuscript. [Comment] All of the above aside, I am unfortunately not convinced that this paper reveals sufficiently new insights for publication in the journal. There are very many example of materials that are similar to those reported here (differences in preparation methods notwithstanding). I am not convinced that the reported values are of sufficient importance for the readership.

[Response]
We thank the reviewer for the comment. It is well-known that the synthesis methods are indeed crucial for the high activity and robustness of the noble metal-free catalysts for highcurrent-density HER. Although MoC and Mo2C were investigated for high-current-density HER catalysts in recent years, the synthesis of an HER catalyst that can work at high current densities (≥ 1000 mA cm -2 ) for weeks is still challenging. As highlighted by Reviewer #1, our synthesized catalyst possesses the almost highest performance in HER activity and stability among the results reported. And to our knowledge, this work is the first to show an HER catalyst that can be working at 1000 mA/cm 2 for two weeks without noticeable degradation. This excellent HER performance benefits from the superfast self-heating synthesis, which not only produces a highly active heterogeneous electrocatalyst but also enhances its stability.
In the following, we will summarize the advantage of self-heating synthesis and highlight the novelty of our work.
(1) Extremely high productivity of synthesis. Although noble-metal-free and high-currentdensity HER catalysts are much demanded in practical applications (Nat. Mater. 18, 1309-1314(2019Nat. Commun. 11, 2940(2020), their fast and scalable synthesis methods have still been very limited. In this work, we develop a fast, self-heating (Joule heating) method using the CNT film as heat source and matrix to in-situ synthesize a robust Mo2C/MoC/CNT HER catalyst in the presence of Mo and C precursors. In this method, the heating and cooling processes rapidly occur in hundreds of milliseconds, and the entire synthesis process only lasts for tens of seconds. The productivity of the self-heating synthesis reaches about 2,000 growth cycles per day, much higher than those of traditional methods such as furnace heating and hydrothermal synthesis.
(2) Novel mechanism for both high activity and high stability. For the improvement of the stability of the HER electrocatalyst, binders would be almost inevitably referred to (Nat Commun 10, 631 (2019); Sci. Adv. 5, eaav6009 (2019)). However, binders would weaken the chemical activity of catalysts and may still fail at high current densities due to corrosion. In our method, owing to the extremely high temperature in the self-heating synthesis, the as-prepared, uniformly dispersed Mo2C/MoC heterogeneous nanoparticles form strong chemical bonds with the CNT heater/electrode. The numerous Mo2C/MoC hetero-interfaces offer abundant active sites with a moderate hydrogen adsorption free energy ΔGH (0.02 eV), and the strong chemical bonding significantly enhances the mechanical stability of the Mo2C/MoC/CNT catalyst. Therefore, high activity and high stability can be achieved simultaneously in our catalyst.
(3) Excellent HER performance at high current densities. Due to the heterogeneous compositing and strong chemical bonding, the Mo2C/MoC/CNT catalyst possesses an ultra-low overpotential of 255 mV at 1500 mA cm -2 in 1 M KOH. The overpotential shows only a very slight change after the catalyst works at 1000 mA cm -2 for 14 days. The HER activity of the catalyst even keeps stable at 3000 mA cm -2 for days. The high activity and high stability suggest that our nonnoble-metal HER catalyst has excellent HER performance, even better than those ever reported in the literature, to our knowledge. The promising HER activity, excellent stability, and high productivity of our catalyst can fulfill the demands of hydrogen production in practical applications.
We believe that our work represents a breakthrough in the synthetic and mechanism innovation of novel heterogeneous catalysts for high-current-density HER applications. In response to this comment, we have added part of the above response in the Discussion section on Page 24, 25 and 26 in the revised manuscript to highlight the novelty of our work. We hope that the reviewer finds our manuscript suitable for publication in Nature Communications now.
[Comment] Title: Ultrafast self-heating synthesis of robust MXene/CNT catalysts for stable highcurrent-density hydrogen evolution reaction

Recommendation: minor revision
In this paper, CNT was used as matrix and heat source to in situ synthesize Mo2C/MoC/CNT catalysts for highly active and stable HER process. The ultrafast heating and cooling rate and short growth time benefited the formation of active sites. The Mo2C/MoC hetero-interface enhanced electron exchange, resulting in the formation of Mo3+ sites serve as the main HER active sites. The strong coupling between the MoxC and CNT matrix ensure the long-term stability at high current densities of catalysts. Here are some suggestions that were shown below for the improvement of manuscript quality.

[Response]
We thank the reviewer for the evaluation of our manuscript and all of the comments. As shown in the following responses, we have supplemented more experiments and theoretical calculations in the revised manuscript to make our conclusion more solid. We hope that the reviewer finds our manuscript suitable for publication in Nature Communications now.
[Comment] 1. To improve the electrode stability, the CNT film was drilled by laser for H2 release channels. Please describe the specific experimental steps of laser drilling.

[Response]
We thank the reviewer for this valuable comment. The CNT films were drilled with many microscale holes by a direct laser writing machine (1064 nm in wavelength). The drilled holes have a diameter of ~ 40 m and a pitch of 800 μm. This kind of holey CNT film effectively releases H2 bubbles during HER, as revealed in our previous work (J. Mater. Chem. A, 8, 17527 (2020)). We have added the above description on Page 26 of the revised manuscript.
[Comment] 2. It was mentioned in this paper that Mo3+ sites served as the main HER active sites. Please provide sufficient evidence that Mo3+ has high HER activity.  (100), and Mo2C/MoC interface are -2.49, -3.86, and -3.09 eV, respectively, which shows that the binding of H on the oxidized MoC (111) surface is the strongest, while for the oxidized Mo2C (100) it is the weakest. The binding of H at the Mo2C/MoC interface is moderate. According to the Sabatier principle, the adsorption of H on a catalyst should not be too weak or too strong. Weak adsorption would lower the capability of hydrogen formation and strong adsorption would restrict the release of hydrogen. The moderate adsorption of H at the Mo2C/MoC interface could result in excellent thermodynamic activity in HER. These results are consistent with the trend of the HER activity in the experiment and thus verify that the Mo at the interface are more HER active than Mo 3+ in MoC or Mo 2+ in Mo2C. [Comment] 3. In the mechanical tensile experiments, whether the improvement of material mechanical strength was related to the rapid rise and fall of joule heating? Please add to prove the comparison sample of pure CNT film heated by joule heating.

[Response]
We thank the reviewer for the valuable comment. We measured the mechanical properties of a pure CNT film before and after the self-heating process. As shown in Fig. R21, after the self-heating, the breaking strain of the pure CNT film is notably reduced while the breaking strength only slightly increases to ~0.32 MPa. In contrast, the Mo2C/MoC/CNT film synthesized by the self-heating process has a much higher breaking strength (~6.87 MPa). These results suggest that the enhancement of the mechanical strength of the Mo2C/MoC/CNT film should be mostly attributed to the strong interaction between Mo2C/MoC and CNTs, which improves the load transfer efficiency inside the film.
In response to this comment, we added Fig. R21 as Supplementary Fig. 36b and the above discussion on Page 19 of the revised manuscript. Fig. R21 | Stress-strain curves of a pure CNT film before and after a self-heating process (30W for 30s and then 135W for 45s).
[Comment] 4. As shown in Fig. 4e, although the f-Mo2C/MoC/CNT catalyst deteriorated after 6 days of test, the potential did not change significantly. Please supplement the data of f-Mo2C/MoC/CNT catalyst tested after 14 days and compare it with the Mo2C/MoC/CNT catalyst prepared by self-heating.

[Response]
We thank the reviewer for this valuable comment. Following the reviewer's suggestions, we tested the long-term stability of an f-Mo2C/MoC/CNT film to compare it with the Mo2C/MoC/CNT film by self-heating. As shown in Fig. R22, the overpotential of the Mo2C/MoC/CNT film increases by only ~47 mV after working at 1000 mA cm -2 for 14 days (336 h), while the overpotential of the f-Mo2C/MoC/CNT film increases by ~344 mV, more than 7 times that of the Mo2C/MoC/CNT film, during the same testing period (14 days). Therefore, the stability of f-Mo2C/MoC/CNT film is much inferior to that of the Mo2C/MoC/CNT film, as we claimed in the manuscript. In response to this comment, we replaced Fig. 4e by Fig. R22 and added the above discussion on Page 17 of the revised manuscript.
[Comment] 5. In supplementary Fig. 23, the f-Mo2C/MoC/CNT catalyst showed the XRD diffraction peak of MoO3 after 6 days of test, while the Mo2C/MoC/CNT catalyst prepared by selfheating did not show the MoO3 peak. Please explain why the self-heating sample has better stability.

[Response]
We thank the reviewer for this valuable comment. The Mo2C/MoC/CNT catalyst has better resistance to oxidation during HER because of its higher crystallinity than f-Mo2C/MoC/CNT film (see Fig. 2b vs Supplementary Fig. 25b and Supplementary Fig. 31 a-b vs Supplementary Fig.  31 c-d). In the fast self-heating synthesis, the growth temperature is very high (~1770 K), which is beneficial for the high crystallinity. In the furnace-heating, however, it is difficult to obtain high crystallinity of the f-Mo2C/MoC/CNT film and simultaneously maintain the Mo2C/MoC composite phase due to the much longer heating process at a lower temperature. Therefore, f-Mo2C/MoC/CNT film has a lower crystallinity and will be gradually oxidized during the high-current-density HER.
In response to this comment, we added part of the above discussion on Page 18 of the revised manuscript.
Reviewer #1 (Remarks to the Author): [Comment] The authors have significantly improved their manuscript and addressed the primary concerns raised: (1) the phase of the Mo2C formed, (2) missing details in the experimental section, (3) the chemical nature of the interface and its importance in explaining the high HER activity and (4) providing additional negative controls to showcase the HER activity. Overall, the manuscript is improved greatly and will be ready for publication after the minor comments below are addressed. I appreciate the authors' efforts in addressing the individual pointers raised previously.
[Response] We thank the reviewer for the positive evaluation on our manuscript and all of the comments. As shown in the following responses, we have supplemented more analyses in the revised manuscript. We hope that the reviewer finds our manuscript suitable for publication in Nature Communications now.
[Comment] Regarding (1), supplementary figure 3 is a very important complement to Fig 2b, which illustrates clearly that the Mo2C is primarily α and β phase, and not as a MXene, as the authors have clarified. This is because the 2D Mo2C MXene (002) peak is very weak and very broad, which is uncharacteristic of 2D layered materials with a well-defined c-lattice parameter. In this regard, I suggest that the authors provide a simple quantitative estimate of the α to β phase ratio through Le Bail XRD peak fitting, which will be very useful for readers to know. [Response] We appreciate this valuable comment. Following the reviewer's suggestion, we quantitatively estimate the content of α-MoC and β-Mo2C by Le Bail and Rietveld XRD peak fitting of the data (Fig. 2b) in the analysis software GSAS. We first imported the crystallographic information files (cif) of α-MoC and β-Mo2C as the reference phases for Le Bail extraction, and then used the Rietveld refinement for further analysis of α-MoC-to-β-Mo2C phase ratio. As shown in Fig. R1, I(obs) is the original data and I(calc) is the Rietveld refined result, both of which fit quite well and the Rwp is 7.1%. After the analysis in GSAS, the fitted weight percentages of α-MoC and β-Mo2C are 59.8% and 40.2%, respectively, corresponding to an α-MoC-to-β-Mo2C molar ratio of about 2.8:1. According to the molar content of α-MoC to β-Mo2C obtained from the Le Bail and Rietveld refinement, we replotted the turnover frequency (TOF) curves, as shown in Fig. R2. At an overpotential of 250 mV, the TOFs of Mo2C/MoC/CNT film, Mo2C/CNT film, and MoC/CNT film are 0.65, 0.30, and 0.22 s⁻ 1 , respectively. These values are identical to the previous TOFs based on the XPS fitting, which validates the highest intrinsic activity of the Mo2C/MoC/CNT catalyst and does not change the conclusion. In response to this comment, we added Figs. R1 and R2 as Supplementary Fig. 3a and Supplementary Fig. 24, respectively. We also added the above description on Page 7 of the revised manuscript.
[Comment] A minor comment here will be to initially first refer to MXene as 2D Mo2C, as this paper is expected to be read by colleagues in the bulk transition metal carbide ceramics field (α and β phase), who might (or might not) be familiar with 2D transition metals and carbides (MXenes). It will also be useful for authors to replace "Tx" with surface terminations since Tx is a MXene terminology and is now hardly referred to in this manuscript. Nonetheless, the authors have very clearly identified the main phases of Mo2C which is very important for future benchmarking.
[Response] We thank the reviewer for these good suggestions. In our revised manuscript, we have referred to MXene as 2D Mo2C, and also replaced "Tx" with the expression of "surface terminations".
[Comment] On (2), the experimental section is now more adequate for readers to reproduce the data.
[Response] We thank the reviewer for the positive evaluation.  [Comment] A minor comment here is to label what element each colored sphere represents in Figure R7.
[Response] We appreciate this comment. As shown in Fig. R4, we labelled the element of Mo and C the colored spheres represent in this figure (re-numbered as Supplementary Fig. 8). [Comment] With regards to (4) the negative controls provides stronger evidence that the authors' materials are indeed intrinsically more active, when normalized to ECSA and when considering intrinsic performance indicators like TOF. Can I check if the HER polarization curves in Figure R9 is normalized to the mass of the active material Mo2C/MoC present? [Response] We thank the reviewer for the positive evaluation. In the original Fig. R9, the mass of the active materials, i.e., Mo2C/MoC on the Mo2C/MoC/CNT film, Mo2C on the Mo2C/CNT film, and MoC on the MoC/CNT film, are 2.1, 2.4, and 2.0 mg, respectively, which have only a little difference. The HER polarization curves shown in the last round of responses to comments had not been normalized to the mass. Following the reviewer's suggestion, we normalized the polarization curves to the mass of the active materials in the three kinds of films (Fig. R5), which also shows the highest activity of the Mo2C/MoC/CNT film and does not change the conclusion. In response to this comment, we added Fig. R5 as Supplementary Fig. 23b. We also added the above description on Page 15 of the revised manuscript.
[Comment] The authors have written an extensive response to all comments by the reviewers. I cannot confidently comment on the authors' response to Reviewer 1's concerns about whether or not an MXene has been formed. I think that the publishers must refer to Reviewer 1 for an updated take on whether he/she is confident in the material characterisation. When it comes to the response to my concerns, I note that the authors have defended the use of the specific capacitance to measure the electrochemical surface area. I do realise that this has been done many times, and is generally accepted. I just think that it comes with some caveats. The accepted value is 20-60 microfarads per square centimetre and generally researchers use as a value of 40. I accept that this is generally done but I have reservations about it. The actual value could be very different from that reported throughout the paper and I would suggest that some acknowledgment of this would add to the paper. Overall, I still have some reservations about the potential impact of the work. Having said that, as long as the authors have satisfactorily addressed the comments of reviewers 1 and 2 (to their satisfaction) I think the paper can probably be accepted.

[Response]
We appreciate the reviewer's evaluation and all of the valuable comments on our manuscript. We also thank the reviewer for her/his acceptance of the use of the specific capacitance to evaluate the ECSA in our work. Following the reviewer's suggestion, we added some acknowledgement of the difference between the actual value and the reported value of the specific capacitance on Page 15 of the revised manuscript as follows, "Cs for a flat surface is generally found to be in a range of 20-60 μF cm -2 , and the value of 40 μF cm -2 is used in this work to calculate the ECSA. The actual value of Cs could be very different from the used value and thus an error may be introduced in the absolute value of ECSA." As the reviewer will see in this round of comments, previous Reviewer #3 has already suggested publication of our manuscript, and previous Reviewer #1 recommended publication after some minor comments are addressed. The new Reviewer #4, also recommended minor revision and only commented on the theory part of our manuscript. We believe that our work will have potential impact for the high-current-density HER applications due to the high productivity of materials synthesis and the synergistic mechanism of heterogeneous electrocatalysts. We hope that the reviewer finds our revised manuscript suitable for publication in Nature Communications now.
[Comment] This is a joint theory-experimental work discussing high current density for HER by carbides/CNT catalysis. The efficiency and stability are quite impressive. Given my expertise I will only comment on the theory part. I recommend minor revision before proceeding: [Response] We thank the reviewer for the positive evaluation on our manuscript and all the comments. As shown in the following responses, we have supplemented more analyses in the revised manuscript to make our work more solid. We hope that the reviewer finds our revised manuscript suitable for publication in Nature Communications now.
[Comment] It is difficult to understand O pz energy center at interfaces' relation to HER activity. I understand its connection to ORR or OER; why this is important for H adsorption energy at surface in HER? The authors suddenly introduced oxidized surface but that is a different system from the ones which the authors are studying in this work. This is quite confusing.
[Response] We thank the reviewer for this comment. The pz-band energy center of the oxygen (ε p z ) could be considered as a good descriptor for the hydrogen adsorption free energy (ΔGH*). According to the DFT calculations, the values of ΔGH* (ε p z ) for the MoC (111)   As discussed in our manuscript, there appear high-valence Mo states (Mo 4+ , Mo 5+ , or Mo 6+ ) in the XPS spectra (Fig. 2d), which should originate from MoOx due to the partial surface oxidation of Mo2C, MoC, or Mo2C/MoC. This result indicates that either Mo2C, MoC, or Mo2C/MoC heterostructure is subject to surface oxidation, and thereby their surfaces are probably oxygenterminated. In addition, the calculated Pourbaix diagram also shows that, under the real reaction condition, the surface of Mo2C is terminated by oxygen, which plays a key role in the catalytic performance of HER in alkaline media (Nat. Commun. 2019, 10, 269). According to the above results, the oxygen-terminated MoC (111) surface, Mo2C (100) surface, and Mo2C/MoC heterostructure are considered for HER in our study.
In response to this comment, we added the above discussion on Page 22 of the revised manuscript.
[Comment] The energy scales the authors care about for H binding energy are very small, e.g. 0.02 eV, but vibrational entropy and solvation energy were not explicitly included. This can introduce over 0.1 or 0.2 eV error already. Can we believe the results at this energy scale? [Response] We thank the reviewer for this valuable comment. In our manuscript, the Gibbs free energy of adsorption hydrogen (ΔGH*) was used to evaluate the HER activity of the catalyst. ΔGH* was calculated using ΔGH* = ΔEH* + ΔEZPE − TΔS, where ΔEH*, ΔEZPE and ΔS are the adsorption energy, zero-point energy change, and entropy change of hydrogen adsorption, respectively. Therefore, the vibrational entropy has been included in our study. According to the reviewer's suggestion, the values of ΔGH* for MoC (111) surface, Mo2C (100) surface, and Mo2C/MoC interface with the implicit solvent environment as implemented in VASPsol were also calculated and shown in Table R1. For comparison, the values of ΔGH* without the implicit solvent environment are also shown in Table R1. The difference in ΔGH* is in a range of ± 0.1 eV with the inclusion of the implicit solvent environment. However, according to the values of ΔGH* with the solvent effect, the Mo2C/MoC interface also shows excellent thermodynamic activity in HER, which does not change our conclusion in the manuscript.
In response to this comment, we added the above discussion on Page 31 of the revised manuscript.  (111) surface (from 12.30 to 12.16 Å), was also utilized to investigate the Gibbs free energy of adsorption hydrogen (ΔGH*). As shown in Table R2, the values of ΔGH* for Mo2C/MoC heterostructure with the compression of MoC (111) surface are almost unchanged. This was confusing too.

[Response]
We apologize for the confusing description of the effect of graphene, which may make the reviewer misunderstand. In the DFT calculations, graphene was used to study the possible chemical bonding between the catalyst and the CNT substrate, rather than to study the activity of the catalyst. The reason for using graphene instead of CNTs is to simplify the model, as graphene has similar properties to CNTs. The binding energies between Mo2C (MoC) and graphene were calculated, indicating that they could be bonded to each other, which results in the high mechanical stability of the Mo2C/MoC/CNT catalysts. However, for the activity of HER, we calculated the Gibbs free energies of adsorption hydrogen (ΔGH*) of MoC (111) surface, Mo2C (100) surface, and Mo2C/MoC interface without the inclusion of the graphene. In real reaction, HER happens at the catalyst active positions exposed to the electrolyte and the CNTs serve as a supporting and conductive substrate.
In response to this comment, we specified the role of graphene on Page 24 of the revised manuscript.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The authors have characterized their materials adequately, including the phases of the Mo2C catalyst. Negative controls after normalizing catalytic data and turnover also supports their hypothesis. From an experimental point of view, the paper is acceptable for publication and I thank the authors for the additional experiments and clarifications to strengthen their manuscript. The current manuscript can be accepted as is, after reviewer #4 evaluates the theoretical aspects of this manuscript.
Reviewer #4 (Remarks to the Author): Comments to "Ultrafast self-heating synthesis of robust nano-carbides/CNT catalysts for stable highcurrent-density hydrogen evolution reaction" I have carefully checked the revised manuscript and responses. The authors have answered my questions and addressed my concerns successfully. I agree with publishing.